Tools incorporating valve assemblies having a plug, such as a ball or dart, and a plug seat, such as a ball seat or dart seat, have been used for a number of different operations in wells for oil gas and other hydrocarbons. These tools may be incorporated into a string of pipe or other tubular goods inserted into the well. The valve assemblies provide a defined location at which the flow of fluid past may be obstructed and, with the application of a desired pressure, a well operator can actuate one or more tools associated with the assembly.
Remotely operated valve assemblies may be used in the treatment of a subterranean formation adjacent to a well. Valves used for this purpose open ports in the tubing to facilitate treatment of a selected area or section of the formation. The treatments are performed by pumping fluid through the wellhead, into the tubing string and out of the selectively opened ports. Examples of such well treatments include acidizing or fracing. Acidizing cleans away acid soluble material near the well bore to open or enlarge the flow path for hydrocarbons into the well. Fracing may occur by injecting fluids from the surface through the wellbore and into the formation at high pressure to create and force fractures to open wider and extend further. The injected frac fluids may contain a granular material, such as sand, which holds fractures open after the fluid pressure is reduced. Such granular materials are not necessarily required, however. While acidizing and fracing are two examples of treatments that may be performed through the valve assemblies, the scope of the present disclosure is not limited to any particular formation treatment(s) and may include any other treatment, such as, without limitation, CO2 injection, treatment with scale inhibitors, iron control agents, corrosion inhibitors or others.
Treatments in multiple-stage production wells may require selective actuation of downhole tools, such as sleeve assemblies, to control fluid flow from the tubing string to the formation. For example, U.S. Pat. No. 7,926,571 entitled Cemented Open Hole Selective Fracing System, which is incorporated by this reference, describes a system using multiple valve assemblies having ball-and-seat seals, each having a differently sized ball seat and corresponding ball. Such ball-and-seat arrangements are operated by placing an appropriately sized ball into the well bore and bringing the ball into contact with a corresponding ball seat. The ball engages on a section of the ball seat to block the flow of fluids past the valve assembly. Application of pressure to the valve assembly, such as through use of fluid pumps at the surface, may create a pressure differential across the valve assembly, causing the valve assembly to “shift” and thereby open fluid flow the sleeve to the surrounding the formation. Other types of plugs such as darts, or any other shape that can be used to selectively operate the valve assemblies, may also be used to seal the seat and facilitate the creation of a pressure differential to shift the valve assembly and open the sleeve, or actuate a different tool, such as a plug and seat actuated flapper valve, associated with the valve assembly.
If the well or tubing contains multiple plug seats, methods, systems or apparatuses must be employed for passing a plug through certain plug seats, including passing through at least some plug seats without actuating any devices associated with such seats. One such method is to use a ball, dart or other plug that is small enough so that it will not seal against any of the seats it encounters prior to reaching the desired seat. For this reason, the smallest ball to be used for the planned operation is the first ball placed into the well or tubing and the smallest ball seat is positioned in the well or tubing the furthest from the wellhead. After the desired treatments are completed, the direction of fluid flow is reversed so that the treating fluids and formation fluids may be produced through the wellhead. Because each plug is smaller than the seats past which it traveled, the plugs simply move with the fluids through the previously passed plug seats and out of the well.
Valve assemblies, which rely solely on the size of the plug and the seat opening for selecting the tool to actuate, significantly limit the number of valves that can be used in a given tubing string. In such systems each ball size is only able to actuate a single valve and, generally, each plug must have a diameter of at least 0.125 inches larger than the immediately preceding plug. Thus, the size of the liner restricts the number of valve assemblies with differently-sized ball seats.
Devices and assemblies have been introduced to increase the number of valve assemblies that may be actuated by a single plug, such as a ball, dart, or other plug. Such devices and assemblies include those described in U.S. application Ser. No. 12/702,169, filed Feb. 28, 2010 and entitled “Downhole Tool With Expandable Seat;” U.S. application Ser. No. 13/423,154, filed Mar. 16, 2012 and entitled “Downhole System and Apparatus Incorporating Valve Assembly With Resilient Deformable Engaging Element;” and U.S. application Ser. No. 13/423,158, filed Mar. 16, 2012 and entitled “Multistage Production System Incorporating Downhole Tool With Collapsible or Expandable C-Ring,” each of which is incorporated herein by reference. The devices, methods, and assemblies described in these applications, however, place one or more plugs downstream of plug seats with openings smaller than the diameter or other cross sectional dimension of the plug. When the fluid flow is reversed, i.e., fluid begins flowing toward the wellhead, such plugs may seat on the back or outlet side of a previously passed plug seat, blocking the reverse flow. The methods for removing such blockages, such as drilling out the tubing string, are both time consuming and expensive. Therefore, there exists a need for cost effective and time efficient devices and/or methods for circumventing such blockages and thereby allowing the flow of fluids from the well bore to the surface.